01 -  In a large bowl, blend the all-purpose flour, granulated sugar, instant yeast, and fine sea salt until thoroughly mixed. 
 02 -  Add warm milk, softened unsalted butter, and egg. Stir until a shaggy dough forms. 
 03 -  Knead the mixture by hand or with a mixer using the dough hook for 7 to 8 minutes until smooth and elastic. 
 04 -  Gently knead in rainbow sprinkles until evenly distributed throughout the dough. 
 05 -  Place dough in a lightly oiled bowl, cover, and let rise in a warm place until doubled in size, approximately 1 hour. 
 06 -  Punch down dough and shape into a loaf. Position seam-side down in a greased 9x5-inch loaf pan. 
 07 -  Cover loaf pan and let rise for 30 to 40 minutes until dough is puffy. 
 08 -  Preheat oven to 350°F. In a saucepan, bring 4 cups water to a simmer and add baking soda carefully. 
 09 -  Lift the loaf gently and dip the top and sides in the baking soda bath for 10 to 15 seconds. 
 10 -  Place loaf back in pan, brush with melted butter, and sprinkle with extra rainbow sprinkles and pretzel salt. 
 11 -  Bake for 30 to 35 minutes until deeply golden brown and sounds hollow when tapped. 
 12 -  Cool loaf in pan for 10 minutes, then turn out onto a wire rack to finish cooling. 
 13 -  Mix powdered sugar with 1 to 2 tablespoons milk and vanilla extract until smooth. Drizzle over cooled loaf if desired.